Imagine, a frigid, distant shadow-region in the far suburbs of our Solar System, where a myriad of twirling icy objects--some large, some small--orbit our Sun in a mysterious, mesmerizing phantom-like ballet within this eerie and strange swath of darkness. Here, where our Sun is so far away that it hangs suspended in an alien sky of perpetual twilight, looking just like a particularly large star traveling through a sea of smaller stars, is the Kuiper Belt--a mysterious, distant deep-freeze that astronomers are only now first beginning to explore. Makemake is a denizen of this remote region, a dwarf planet that is one of the largest known objects inhabiting the Kuiper Belt, sporting a diameter that is about two-thirds the size of Pluto. In April 2016, a team of astronomers announced that, while peering into the outer limits of our Solar System, NASA's Hubble Space Telescope (HST) discovered a tiny, dark moon orbiting Makemake, which is the second brightest icy dwarf planet--after Pluto--in the Kuiper Belt.
So here you have two definitions of a blue moon but the one for a calendar blue moon does not describe the true meaning of a blue moon. Here's why:
A moon is defined as a natural satellite in orbit around another body that, in turn, is in orbit around its Star. The moon is kept in its position by both its own gravity, as well as its host's gravitational grip. Some planets have many moons, some have only a small number, and still others have none at all. Several asteroids inhabiting our Solar System are circled by very small moons, and some dwarf planets--such as Pluto--also host moons.